It is known that a conventional fuel injection system for a combustion engine includes a fuel rail and a plurality of electrically controlled fuel injectors, which are hydraulically connected with the fuel rail by means of respective feeding conduits. Each fuel injector generally includes a fuel inlet, a fuel outlet and a movable needle which repeatedly opens and closes the fuel outlet. When the needle is in an open position, fuel is injected under pressure into a cylinder of the engine. The movable needle is actuated with the aid of a dedicated actuator, typically a solenoid actuator or a piezoelectric actuator, which is driven by an electric circuit controlled by an Engine Control Unit (ECU). The ECU operates each injection pulse by generating an electric opening command acting upon a control valve and causing the needle to open the fuel injector, and a subsequent electric closing command, causing the needle to close the fuel injector.
The timing of the opening and closing electric commands is also controlled by the ECU, which determines two key parameters for each injection pulse, namely an electric Energizing Time (ET) and an electric Dwell Time (DT). The electric Energizing Time (ET) is the time between the instant in which the electric opening command of an injection pulse is generated, and the instant in which the electric closing command of the same injection pulse will be generated. The electric energizing time is generally determined by ECU as a function of the quantity of fuel to be injected in the course of the injection pulse, taking into account the value of the pressure inside the fuel rail. The Dwell Time (DT) indicates the time interval included between two consecutive injection pulses, namely the time interval between the end of the Energizing Time (ET) of a first injection pulse and the Start of Injection (SOI) of a second consecutive injection pulse.
In order to improve the characteristics of exhaust emissions and reduce combustion noise in engines, particularly in Diesel engines having a common-rail fuel injection system, so-called multiple fuel injection patterns are adopted. In a multi-injection pattern the fuel quantity to be injected in each cylinder at each engine cycle is split into a plurality of injections. More specifically, in a multi-injection pattern, for each engine cycle, a train of injections is performed by each injector typically, starting from a pilot injection and following with a main injection, which gives all or most of the torque in an engine cycle, eventually terminating with after and post injections.
The number of injections of the train of injections and their timing is dependent on the combustion mode and is determined by an Electronic Control Unit of the engine. The Energizing Time and Dwell Time values are generally predetermined with reference to an injection system having nominal characteristics, i.e. with components having no drifts and mapped in a data carrier or memory associated with the ECU. Reducing the Dwell Time (DT) between two consecutive injections below a proper critical value causes the hydraulic fusion of the pilot and main injection, a condition also referred as Injection Quantity Fusion (IQF). An IQF strategy is able to give benefits in terms of Brake Specific Fuel Consumption (BSFC) and/or Combustion Noise (CN) and/or Soot emissions, depending on the calibration used. In particular, a pilot injection before a main injection is an enabler for a better fuel spray atomization and therefore increases combustion efficiency.
Even better than an IQF strategy, is a condition such as Zero Hydraulic Interval (ZHI) between a pilot injection and a main injection, namely a condition in which there is no interval between the hydraulic closing of the needle of the injection after a pilot injection and the hydraulic opening of the needle for a main injection. ZHI may be the best strategy for having the same IQF benefits above mentioned but also very stable injection hydraulics as in standard injection patterns. However, ZHI is a critical and difficult condition to reach and maintain because the electrical Dwell Time (DT) range for controlling the ZHI is very narrow and therefore it is difficult to maintain it along engine life due to disturbances such as injector's aging drift.
In fact, a calibrated nominal electrical DT value is actually used for managing the ZHI strategy, this electrical DT value being held constant for all the rail pressure levels. However the use of a constant electrical DT value is not sufficient for ensuring the correct strategy actuation versus all the engine working conditions and versus ha are drift, such as for example injector's aging drift.
It is, therefore, desirable to provide a strategy that allows the injector to operate in a Zero Hydraulic Interval (ZHI) condition throughout the life of the injector. It is also desirable to reach the above result without using complex devices and by taking advantage from the computational capabilities of the Electronic Control Unit (ECU) of the vehicle.